The French bean (
Phaseolus vulgaris) its domestication started around 6000 years ago in central and south America, which eventually lead to its spread throughout the world. it is an important crop of cooler tropics (
Chatterjee and Bhattacharyya, 1986). The crop is cultivated in a variety of agroecosystems around the world, ranging from tropical, sub-tropical and temperate climates
(Popelka et al., 2004). It is primarily grown and consumed in developing Latin American, African and Asian countries. It is the most essential legume for direct human consumption on the planet.
Beans’ ability to fix nitrogen in the soil, as legumes, contributes significantly to the enhancement and long-term survival of soil fertility. As a result, they’re used in crop rotation, as well as cover crops and green manures
(Stoddard et al., 2010). The states of Maharashtra, Jammu and Kashmir, Himachal Pradesh, Uttar Pradesh, Tamil Nadu (Nilgiri Hills, Palani Hills), Kerala (Parts of the Western Ghats), Karnataka (Chickmagalur Hills) and West Bengal (Darjeeling Hills) are the major producers of common bean in India
(Choudhary et al., 2018). The crop can withstand daytime temperatures of 35°C on occasion, but flower abortion is common. Frost destroys the plant if the temperature drops below 10°C
(Messiaen et al., 2004).
Fungal diseases, in general, have a negative impact on crop development and productivity
(Parveen et al., 2021). Phoma exigua var.
exigua desmaz (syn.
Ascochyta phaseolorum Sacc.) induces Ascochyta blight
(Parveen et al., 2019). The pathogen can survive on the seed for up to two years and it can also spread from diseased plants and old plant tissues
(Srivastava et al., 2020). The fungus can spread across the plant and it is also spread
via seed
(Kumar et al., 2020a). The pathogen thrives in conditions that are cool and humid
(Hanson et al., 1993). When disease severity is high, the plant loses leaves and pods under favorable conditions
(Bardas et al., 2008). Phoma is a large genus of fungi that can be found in a variety of ecological niches and is found all over the world. Several
Phoma species are also quarantine-relevant, posing significant challenges to organizations responsible for plant health quarantine control
(Aveskamp et al., 2008).
Symptoms
The symptoms Ascochyta blight first appear on the leaves (Fig 1A). These are black, concentric, zonate lesions that are 1-3 cm in diameter and may produce small black pycnidia later on
(Liu et al., 2016). These dark to black lesions can affect the peduncle, petiole, node and pod, causing stem girdling and plant death. The fungus has the ability to spread across the plant in a systemic manner. During severe epidemics, premature leaf fall can occur and the fungus is seed-borne (
Urinzwenimana, 2017). The disease usually appears during the early stages of pod development and becomes more severe as the pods mature. Small, circular, dark-brown spots appear first on the leaves, then the leaves and pods. The lesions then turn a dark brown to black colour and have clearly visible concentric rings. The concentric ring lesions range in size from 10 to 30 mm in diameter and are primarily filled with small black pycnidia. On the leaves, lesions may coalesce and cover the majority of the leaf surface area. This causes dark grey to black concentric rings to form on the leaves, which then coalesce and the diseased tissues’ centres fall out over time. Sunken lesions that girdle the stem cause breakage and eventual death of the sections above the diseased region, resulting in stem attack. If pods are present at the time of the attack or if the pathogen spreads to emerging or existing pods as the disease progresses, they become infected and lesions with concentric rings emerge as shown in Fig 1B (
Luthra and Bedi, 1932). The canopy of severely infected plants has a tattered appearance, with a significant loss of leaves (senescence). Sunken lesions on the pods are divided into zones with a pale centre and dark edges and are normally covered by several pycnidia
(Kumar et al., 2020b).
Pod drop caused by the disease has been confirmed in other parts of the world, with a negative yield potential for the crop. Lesions on pods may also be a cause of seed infection contamination. Infected seeds have a brown to black discoloration, making them unsuitable for seed and grain markets
(Kumar et al., 2020b). Seedlings developed from infected seeds sometimes develop dark brown to black sunken lesions on the stems and cotyledons, which senesce prematurely, resulting in stunted plant growth. The severity of the disease is weather-dependent, so high yield loss is normal in cold, wet and high-relative-humidity environments (
Tivoli and Banniza, 2007).
The pathogen
The pathogen is linked to the development of pseudothecia at the end of the cropping season, which helps the pathogen to survive in plant debris (
Tivoli and Banniza, 2007). In culture,
Phoma exigua isolates create a hyaline, septate, submerged mycelium. Spores are typically made up of two cells (
Zaumeyer and Meiners, 1975). At 21°C, sporulation and germination are at their peak, while mycelial development is at its peak at 24°C. Temperatures over 30°C destroy the fungus (
Schwartz, 1989;
Schwartz and Steadman, 1989). Pycnidia, which are 60-150 mm in diameter, are produced by the fungus (Fig 2). The pycnidia of
Phoma exigua var.
diversispora measure 160 by 120 nm and the conidia are 6.8 by 2.7 nm (
Urinzwenimana, 2017).
Most conidia are single-celled
(Boerema et al., 1981). Conidia were cylindrical to oval in shape, allantoid, hyaline, pale yellow to brown in colour, typically one-celled and 2 to 3 × 5 to 10 µm in length
(Marin-Felix et al., 2019). Some diagnostic characteristics of pathogens appeared on French beans are revealed in Table 1.
In all above-mentioned cases, pycnidia on the leaves, when not present at time of collection will develop within 48 hours after incubating detached leaves with leaf spots in Petri dish on moist filter paper. The type species of
Phoma is only distantly related to the other members of this genus, but relatively close to
Ascochyta pisi, the type species of the older name
Ascochyta (Aveskamp
et al., 2010). Since earlier, the genera
Phoma and
Ascochyta, both classified in the order Pleosporales of Ascomycota, have already been considered as closely related (de
Gruyter et al., 2009) although also having some differences mentioned in Table 2.
There is a convention of using a single generic name, based on priority but regardless of whether the genus is “anamorphic” or “teleomorphic”. This classification is used for all unambiguous monophyletic phylogenetic lineages
(Aveskamp et al., 2010). The nine
Phoma sections have teleomorph relations described in the genera
Didymella, Mycosphaerella, Leptosphaeria and
Pleospora (
Boerema, 1997) indicating that
Phoma anamorphs represent a polyphyletic group (Table 3 and 4).
Favouring epidemiological factors
The pathogen attacks the plant at various stages of development, resulting in yield losses of up to 100% when infected seeds are used for sowing and the climatic conditions are favourable for pathogen survival
(Allen et al., 1996). High humidity, constant rains with winds and cool to moderate temperatures are conducive to infection by
Phoma exigua var.
diversispora (Boerema et al., 1981).
Survival and spread of the disease
When contaminated seeds are used for sowing, the pathogen is known as a wound-pathogen in various plant species and can cause extreme dumping. It has a polyphagous life cycle that allows it to live for many seasons in plant debris (
MacLeod and Sweetingham, 1999). If air currents disperse ascospores produced in the perithecia on plant debris after short rains, they can fly a few kilometers from the infected to the new host plant, though most land only a few meters from the source (
Tivoli and Banniza, 2007). After rainfall, the pseudothecia release ascospores, which are then spread over long distances by the wind. This explains why plant debris is such an important source of primary inoculum for Ascochyta blight production in subsequent seasons. The pathogen targets the bean plant’s pods, cotyledons, growing hypocotyl, leaves and other aerial parts. Long-distance spread of the disease is primarily accomplished by infected seeds, while short-distance spread is accomplished by pycnidiospores carried by the wind and raindrops.
Management strategies of ascochyta blight
There have been many attempts to regulate Ascochyta, but none have been successful. As part of integrated pest management, it has been possible to handle Ascochyta to some degree by using a combination of control options (cultural, chemical and biological) (
Buruchara, 2006). However, the use of resistant bean genotypes is the single most effective and realistic management strategy, particularly for resource-strapped farmers (
Hall and Nasser, 1996). As only incomplete resistant varieties are available against this disease, some agronomic practices should be applied to control and reduce Ascochyta blight incidence wherever possible
(Ouji et al., 2022). Crop rotation, wide plant spacing, clean seed planting, chemical seed treatment and foliar fungicide application are some of the control measures used (
Schwartz, 1989;
Schwartz and Steadman, 1989). Crop replacement with non-host crops for at least two years, use of uncontaminated approved plants, intercropping, field sanitation and destruction of infected crop residues may all help to contain the pathogen
(Khan et al., 2013; Kumar et al., 2014). Since the fungus is dispersed in presence of water, fields should not be entered for cultivation or pesticide applications while the plants are wet, preventing unwanted movement in infested fields and reducing disease spread. This aids in the preservation of hygiene both on and off the farm
(Moore et al., 2013).
Use of tolerant or resistant varieties
Host plant resistance is the most cost-effective, eco-friendly and recommended form of controlling bean diseases
(Wahome et al., 2011). Bean breeding has aimed to create French bean cultivars that are more resistant to biotic stresses
(Sharma et al., 2007). There are French bean varieties that are resistant or tolerant to
Ascochyta. Current cultivars only have a low level of resistance to the pathogen, which is quickly broken down
(Gan et al., 2006). This is because the pathogen’s sexual stage (teleomorph) leads to the development of new races or pathotypes
(Bretag et al., 2006; Gan et al., 2006). Because of the many physiological races of
Phoma exigua and the pathogen’s high variability, host resistance alone may not be enough. This is because varieties that are resistant to one race may be susceptible to another and tolerance sometimes breaks down when favourable conditions for disease development prevail
(Duc et al., 2014). Many farmers have been forced to use foliar fungicides as a result of this. In order to control Ascochyta blight, a combination of genetic resistance and an effective foliar fungicide application program has been found to be effective
(Gossen et al., 2011).
Biological control
The use of biocontrol agents has become a safer choice for the management of this disease. Several researchers are currently working in this field to discover environmentally friendly biocontrol agents having stronger capabilities against this phytopathogen
(Kumar et al., 2020b).
Seed treatment with
Trichoderma harzianum and
Bacillus subtilis could effectively be used for controlling the infection of
Phoma exigua (Bhattacharjee
et al., 2019)
. Few other fungal antagonists such as
Chaetomium globosum, Trichoderma viride and
Acremonium implicatum are also giving promising results
(Kumar et al., 2020b). Some botanicals extract of
Allium cepa and
Maesalan ceolata var.
goulun gensis showed effective results against test pathogen
(Rai et al., 2000; Choudhury et al., 2018).
Chemical management
The key routes of introduction of Ascochyta blight to a healthy crop are seed-borne inocula as well as infected or contaminated crops
(Gan et al., 2006). The use of wide spectrum fungicides in seed treatment can help to reduce the number of fungal pathogens on the seed or in the soil. Seed dressings, on the other hand, can only protect emerging seedlings from Ascochyta blight transmitted via the seed. This means that seed dressing alone is ineffective in protecting emerging seedlings from Ascochyta blight inoculum splattered by rain
(Markell et al., 2008). Synthetic chemicals have been related to the production of pathogen resistance to the chemicals, pollution, human health hazards and becoming prohibitively costly for smallholders
(Pinto et al., 2010).
